1. Field of the Invention
This invention relates to a countercurrent, cooling crystallization and purification method for a multi-component molten mixture. More particularly, this invention relates to a method for separation in pure form of one component from the multi-component molten mixture by employing a cooling-type crystallizer or crystallizers and a vertical, melting-type purifier in combination.
2. Description of Prior Art
To separate a component, in pure form, out of the multi-component mixture, a unit operation such as distillation or extraction is usually carried out. However, the former is disadvantageous in that it consumes a lot of heat energy, and the latter has such a disadvantage that the operation is very complicated and yet the purity of the product is not always satisfactory. In this connection, it is to be noted that if a desired component, for example component A, can be crystallized in pure form, at a certain concentration, upon cooling of the multi-component molten mixture, the desired separation can be attained by crystallization.
The separation procedure by crystallization, especially in commercial operation, will now be explained in more detail. The starting multi-component molten mixture is cooled to crystallize the component A out of the mixture and the crystallized component A is subjected to solid-liquid separation to obtain the pure component A in the form of crystals. As these operations proceed, the contents of the component, for example component B, other than the component A will be increased in the mother liquor from which the component A has been separated. To further recover the component A which is still contained in the mother liquor, the mother liquor is further cooled to a temperature lower than the crystallization temperature of the first stage. Therefore, the crystallization is usually carried out in such a manner that (1) a plurality of unit crystallizers are arranged in series to form a continuous multi-stage crystallization system or (2) a horizontal, agitated crystallizer is operated continuously.
In the former case (1), the temperatures of the crystallizers are different from each other and the crystallizers are arranged in order of temperature, i.e., from higher temperature to lower temperature. The liquid to be treated flows in that order. In this method, however, the crystals formed are suspended in and mixed with the mother liquor in the respective crystallizers and the crystals are transported along with the mother liquor to the crystallizers of the succeeding stages. As a result, in the crystallizer of the final stage, the crystals of the component A coexist with the mother liquor whose concentration of the component B is highest among the crystallizers. Accordingly, the mother liquor remaining on and adhering to the crystals after separation operation of the crystals by a solid-liquid separator has a high concentration of the component B. Thus, it is difficult to obtain the component A in pure form.
In this connection, it is to be noted that, in the multi-stage crystallization system of the above-mentioned type, the crystals of component A are preferred to contact the mother liquor containing component B countercurrently. With this arrangement, the product crystals can be obtained from the crystallizer of the first stage in which the content of component B is lowest, so that the contamination of the product with the component B can be minimized.
Such countercurrent arrangement is widely employed in the operation of crystallization. In the simplest example of the arrangement of this type, the feed liquid is generally allowed to flow sequentially to the crystallizers in series and the crystals formed in each of the crystallizers are separated by a solid-liquid separator provided between the respective crystallizers so as to feed the obtained crystals to the crystallizer of higher temperature and let the mother liquor flow to the crystallizer of lower temperature. This method is desirable in that the transportation direction of the crystals is opposite to that of the mother liquor flow so that the crystals may sequentially come into contact with a mother liquor having a higher content of component A in the crystallizer of higher temperature and the component B adhering to the crystals may be diluted by such a mother liquor. This method, however, is not desirable in cost because it requires an expensive solid-liquid separator such as a filter or a centrifugal separator and it takes considerable cost for operation and maintenance of the separator.
The conventional method (2) is carried out for example by an apparatus comprising a crystallizer provided with a ribbon blade and a horizontal cooling jacket. In this apparatus, crystals produced by cooling a feed liquid are continuously transported in one direction and a mother liquor is allowed to continuously flow in the opposite direction. However, the intended countercurrent contact between the crystals and the mother liquor is not always attained because the crystals are suspended in the liquor and carried along with the liquor. This apparatus has another disadvantage that it should be long enough to acquire desired temperature difference between the product recovering portion and the portion containing the liquor of low purity, i.e., to attain temperature difference between the melting point of the product and the crystallization point of the mother liquor containing comparatively large amount of impurities, because it requires a distance of 2 m to acquire temperature difference of 1.degree. C. Thus, the apparatus requires a considerably large space for the installation thereof. The apparatus further has a disadvantage that the transportation of the crystals is not carried out uniformly in the crystallizer so that the relative contact between the cold crystals and the hot mother liquor cannot be stable and the amount of crystal formation is varied, rendering the operation unstable.
In these conventional methods, the crystals obtained from the crystallizer or crystallizers are generally fed to a vertical, melting-type purifier so as to be purified there or subjected to a separating operation for example by a centrifugal separator to obtain the crystals as products. In the former case, the purifier is combined with the crystallizer or crystallizers in series to effect continuous separation operation. The purifier employed in these conventional methods is not provided with a settler or clarifier for separating the mother liquor from crystals so as to feed the liquor to the crystallizer or crystallizers of lower temperature. For this reason, the crystals formed are inevitably transported along with the liquor and the separation efficiency is not high. To solve this problem, it is proposed to provide a settler for separating the crystals from the mother liquor.
In the latter case, the mother liquor cannot completely be removed from the crystals by the centrifugal separation etc. and it cannot completely be eliminated even after washing. Thus, there is a limit in purity of the crystals which can be attained by this method. More specifically, a cake obtained by the centrifugal dehydration has many voids between grains of the crystals. The grains are strongly combined with each other and the voids are filled with a gas, so that a washing liquid which is accelerated by a centrifugal force is caused to pass by the grains without having sufficient contact with the grains and without penetrating into contact surfaces of the grains. Thus, there is a limit in purity attainable by this method, and it is required to repeatedly carry out recrystallization for improving the purity of the obtained crystals.